1. Field of the Invention
The present invention relates to an image reading optical system and an image reading apparatus, and more particularly, to an image reading optical system and an image reading apparatus which are suitable for an image scanner, a digital copying machine, or the like, which needs image reading with large aperture and stable contrast performance.
2. Description of the Related Art
Conventionally, there is an image reading apparatus for reading image information on a platen, which uses a linear image sensor including a plurality of light receiving elements arranged in a main scanning direction. An image of the image information is formed on the linear image sensor by an imaging optical system and a relative position between the document and the linear image sensor is changed in a sub-scanning direction. Thus, the image reading apparatus reads the image information.
FIG. 17 is a schematic diagram of a conventional integrated carriage type scanning image reading apparatus. In FIG. 17, a beam emitted from an illumination light source 81 directly illuminates a document 87 placed on a platen glass 82. An optical path of a reflected beam from the document 87 is folded in a carriage 86 by first, second and third turn back mirrors 83a, 83b and 83c in turn, and hence an image of the image information on the document 87 is formed on a linear image sensor 85 by an imaging lens (imaging optical system) 84.
Then, the carriage 86 is moved in a direction of the arrow A (sub-scanning direction) by a motor 88 for scanning in the sub-scanning direction, and hence the image information on the document 87 is read. The linear image sensor 85 of FIG. 17 has a configuration in which a plurality of light receiving elements are arranged in one-dimensional direction (in the main scanning direction).
FIG. 18 is an explanatory diagram of a basic configuration of a reading optical system of the image reading apparatus illustrated in FIG. 17. In FIG. 18, the imaging optical system 84 is disposed, and the linear image sensor 85 is constituted of linear image sensors 85R, 85G and 85B for reading red (R), green (G) and blue (B) colors, respectively. FIG. 18 further illustrates reading ranges 87R, 87G and 87B on the document 87 to be read by the linear image sensors 85R, 85G and 85B, respectively.
The document 87 is scanned in the sub-scanning direction, so that the same part is read by the linear image sensors for different colors with a certain time interval. In the above-mentioned configuration, if the imaging optical system 84 is constituted of an ordinary refracting system, axial chromatic aberration, lateral chromatic aberration, or the like occurs. Therefore, with respect to the linear image sensor 85G as a reference, defocus or a positional shift occurs in line images formed on the linear image sensors 85B and 85R. Therefore, when the color images are overlaid to reproduce the image, color blurring or color drift becomes conspicuous in the image. In other words, when a performance with large aperture and high resolution is required, the requirement cannot be satisfied.
On the other hand, there is a technology for satisfying the above-mentioned requirement by using an anamorphic optical system that is asymmetric in the main scanning direction and in the sub-scanning direction in an optical system on the precondition of the linear image sensor. In particular, in a non-coaxial optical system among the anamorphic optical systems, it is possible to form an optical system in which aberration is sufficiently corrected by utilizing a concept of reference axis and forming an asymmetric and aspheric element surface.
This non-coaxial optical system is called an off-axial optical system, which is defined as an optical system including a curved surface in which a surface normal of the element surface at an intersection with the reference axis is not on the reference axis (off-axial curved surface) when considering the reference axis along a light beam passing through a center of the image and a center of a pupil. In this case, the reference axis has a folded shape.
This off-axial optical system has an element surface that is usually a non-coaxial surface, and vignetting does not occur on a reflection surface. Therefore, it is easy to constitute an optical system using a reflection surface. In addition, it is easy to form an integrated type optical system by a method of integrally molding a surface on which an optical path can relatively freely designed.
Japanese Patent Application Laid-Open No. 2006-259544 discloses an off-axial optical system in which two off-axial reflection surfaces are combined. According to this optical system, a mold and a molding machine, which otherwise tend to be expensive, can be integrated, so that manufacturing cost can be significantly reduced. Thus, as illustrated in FIG. 19, it is possible to realize the image reading apparatus having a small size and a small number of components.
In FIG. 19, a light source device 1 is constituted of a fluorescent light, an LED array, or the like. A document (object) 7 is placed on a platen glass 2. FIG. 19 further illustrates a first reflection mirror 3a, a second reflection mirror 3b and a third reflection mirror 3c. An imaging optical system 94 for image reading (off-axial optical system) forms an image of the beam based on image information on the document 7 on a linear image sensor 5 as a reading unit.
In the imaging optical system 94, the surface normal at a reflection point of a reference axis light beam is not on the reference axis. A free-form surface reflection member 4a in which a plurality of reflection surfaces having a free-form surface shape are formed in an integrated manner is disposed to be opposed to a flat reflection member 4b having a flat reflection surface. The image reading apparatus further includes an aperture stop SP disposed on a flat reflection surface R2 of the flat mirror member 4b. For instance, the aperture stop is formed by attaching a black color resin sheet member to the flat mirror surface or by other such methods. The linear image sensor 5 (light receiving unit) constituted of a CCD or the like is disposed at a position corresponding to an image plane. A carriage 6 (case) houses the individual members 1, 3a, 3b, 3c, 94, 5, and the like.
Here, an arranging direction of pixels of the linear image sensor 5 (X direction perpendicular to the drawing sheet) is regarded as a main scanning direction, and a direction perpendicular to the main scanning direction (Y direction in the drawing sheet) is regarded as a sub-scanning direction. A propagation direction of the beam is regarded as a Z direction. In this case, an XZ plane is a main scanning cross section, and an YZ plane is a sub-scanning cross section. The beam emitted from the light source device 1 illuminates the document (object) 7 placed on the platen glass 2, and the beam from the document 7 enters an off-axial reflection surface R1 of the off-axial reflection surface member 4a via the first reflection mirror 3a, the second reflection mirror 3b and the third reflection mirror 3c. 
Then, the beam reflected by the off-axial reflection surface R1 enters the flat reflection surface R2 of the flat mirror member 4b so as to be reflected at an acute angle. After that, the beam enters an off-axial reflection surface R3 different from the off-axial reflection surface R1 and is reflected. Then, an image of the beam is formed on the linear image sensor 5. Note that, in this case, individual reflection surfaces fold the optical path in the sub-scanning cross section. Further, the relative position of the carriage 6 to the document 7 is changed in the sub-scanning direction (the direction of the arrow A), so that image information on the document 7 is read in a two-dimensional manner.
In order to compactly constitute the image reading apparatus, the first reflection mirror 3a, the second reflection mirror 3b and the third reflection mirror 3c fold the optical path. The imaging optical system 94 also contributes to folding of the optical path. In this conventional example, an f-number (Fno) is designed to be 6.0, a magnification is designed to be 0.11, and an object height is designed to be 150 mm. FIG. 22A shows modulation transfer function (MTF) depth characteristics at 60 lines pair/mm on the image plane. A solid line indicates MTF in the sub-scanning direction (S_MTF). A broken line indicates MTF in the main scanning direction (M_MTF). A center 0 of a horizontal axis is a focusing position.
In the imaging optical system 94, the optical path is folded in substantially a Z shape by off-axial surfaces so that decentering aberrations generated on the off-axial reflection surfaces can be easily canceled by each other. Further, the flat mirror member is used so as to fold in a Σ shape without affecting the aberration. Thus, in spite of a simple configuration of one flat mirror member and one off-axial reflection surface member, good imaging performance is obtained.
In recent years, image reading apparatus have been required to support higher speed, and the optical system has been required to have larger aperture. The conventional off-axial optical system of FIG. 20 is caused to have large aperture at the f-number of 4.0 as an optical system illustrated in FIG. 21. The MTF depth characteristics of this optical system are shown in FIG. 22B. In general, the large aperture optical system has a high MTF at the focusing position, which is rapidly decreased in a defocused position. In addition, when the aberration is large, the MTF is low even in in-focus state and is further decreased by a focal point shift.
In particular, in the anamorphic optical system including the off-axial optical system, characteristics in the main scanning direction are not the same as characteristics in the sub-scanning direction. Therefore, as shown in FIGS. 22A and 22B, the MTF in the sub-scanning direction indicated by a solid line indicates a high MTF value at a position 0 of the in-focus state and is rapidly decreased when the focal point shift occurs. In the main scanning direction indicated by a broken line, the MTF value of aberration is lower than that in the sub-scanning direction even in the in-focus state and is further decreased by the focal point shift. FIG. 23 illustrates wavefront aberration in the focusing position of FIGS. 22A and 22B. In general, an aberration amount is larger as being closer to a periphery, and hence the aberration amount is increased when a larger aperture is achieved.
If the focal point shift occurs in the image reading apparatus, not only image deterioration but also various problems occur. For instance, a process for sharpening an image is optimized at the time of shipment from the factory. Therefore, if the focal point shift from the state occurs, an appropriate process cannot be performed, so that an image cannot be sharpened. Therefore, it is desired that the MTF value be always stable. As an index of whether or not the MTF value is stable, the following Equation 1 can be used.MTF stability MS=((highest MTF)−(lowest MTF))/((highest MTF)+(lowest MTF))(%)  Equation 1
In the conventional optical system shown in FIG. 22A, the stability MS is 4.6% at a position within a range of ±1 step of the focusing position. Here, “1 step” is defined as equivalent of 0.025 mm. Therefore, the range of ±1 step of the focusing position corresponds to a movement range of an imaging plane as ±0.025 mm from the focusing position. It is preferred that the stability be 8% or lower so that the above-mentioned problem does not become conspicuous. In the conventional optical system shown in FIG. 22B, the stability MS is 14.4% at a position within a range of ±1 step of the focusing position and is required to be improved.
The focal point shift may be caused by various phenomena. For instance, if the ambient temperature of the image reading apparatus is extremely high or low, the optical element may be deformed, so that the focal point shift may occur. Other than that, if vibration in the installation or transportation is large, a positional shift may cause the focal point shift. At present, in order not to generate the focal point shift described above, there are taken measures such as air conditioning or packing for installation environment or vibration. However, along with economic growth in developing countries, there may be a case where the apparatus is used in an unexpected installation environment or a case where the apparatus is transported in bad road conditions.
Therefore, taking measures against the focal point shift is important for the image reading optical system. As measures against the focal point shift, there are measures such as incorporation of a focus adjustment mechanism, and the like. However, in the image reading apparatus, the carriage moves at high speed as described above, and hence it is difficult to incorporate a weight-increasing mechanism such as the focus adjustment mechanism. Therefore, it is necessary to realize the optical system having a small variation of contrast performance even if the focal point shift occurs.
On the other hand, there is a technology enabling to reduce a variation of contrast performance even if the focal point shift occurs. Japanese Patent Application Laid-Open No. H09-288254 discloses a technology for correcting the contrast performance by an optical phase changing filter with an N- (2-, 3-, . . . ) fold symmetry including a phase lead area having a phase lead action of leading a phase of a wavefront of an incident beam and a phase delay area having a phase delay action of delaying the phase of the wavefront of the incident beam. The N-fold symmetry means that the same shape is obtained before and after rotation by an angle of 360/N degrees about the reference axis. In this case, with respect to the plane that includes the surface normal at the center of the incident beam and is perpendicular to the above-mentioned symmetric plane, one side is regarded as the phase lead area, while the other side is regarded as the phase delay area.
The technology disclosed in Japanese Patent Application Laid-Open No. H09-288254 can be effective in a camera or the like, which uses a general imaging optical system disposed in a rotationally symmetric manner about the optical axis. In other words, it is possible to provide an imaging optical system having little variation of the contrast performance due to the focal point shift as a general imaging optical system disposed in a rotationally symmetric manner.
However, the above-mentioned technology is not sufficiently effective in an image reading apparatus that uses an anamorphic imaging optical system for forming an image of a slit area (imaging optical system having different cross section shapes between the main scanning direction as a longitudinal direction of the slit area and the sub-scanning direction perpendicular to the main scanning direction). In other words, an imaging optical system having little variation of the contrast performance due to the focal point shift cannot be provided as an anamorphic imaging optical system.